Popup: it would be great if it would be it's own window rather than just a popup that automatically closes when you click anywhere
Thumbnail: when you want to edit the default settings of a brush, you can easily destroy your save files by just forgetting to insert a thumbnail -> resulting in a white image for your brush. since it is now the default value, there is also no easy way to reset it besides going into the files. A easy workaround would be to call the "Fill preset area with current icon" whenever a brush is selected. this would also make it easier to know which brush you are editing right now.

Wrong forum? I guess you wanted it in the Krita forum?

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